Curable resin composition, multilayer printed circuit board manufactured by using the composition, and method for the production thereof

ABSTRACT

A multilayer printed circuit board having resinous insulating layers and conductor layers alternately superposed on a circuit board with ample adhesive strength, a method for the production thereof, and a curable resin composition useful for the formation of resinous insulating layers are disclosed. The manufacture of the multilayer printed circuit board is accomplished by applying the curable resin composition to the surface of conductor layer of the circuit board, thermally curing the applied layer thereby forming resinous insulating layer, then boring a through-hole in the circuit board, treating the resinous insulating layer with a coarsening agent thereby imparting undulating coarsened surface thereto, subsequently coating the surface of resinous insulating layer and the inner surface of the through-hole with a conductor layer as by electroless plating, and thereafter forming a prescribed circuit pattern in the conductor layer. The curable resin composition comprises (A) epoxy resins and (B) an epoxy resin curing agent as essential components thereof and optionally contains a rubber component and a filler capable of being decomposed or dissolved by a coarsening agent. This composition contains in combination a bisphenol A type epoxy resin having an epoxy equivalent of not less than 400 and an epoxy resin having an epoxy equivalent of less then 400.

BACKGROUND OF THE INVENTION

1. 1. Field of the Invention

2. This invention relates to a curable resin composition which, in amultilayer printed circuit board having conductor circuit layers andinsulating layers alternately built up or superposed on a substrate, iscapable of forming insulating layers excelling in adhesiveness withconductor layers and also excelling in resistance to heat, a multilayerprinted circuit board manufactured by using the curable resincomposition, and a method for the production thereof.

3. 2. Description of the Prior Art

4. For the production of a multilayer printed circuit board, a methodwhich comprises laminating a plurality of circuit boards having aprescribed circuit pattern formed in advance thereon through the mediumof prepregs as adhesive insulating layers, pressing the laminate thusobtained, drilling and making plated through-holes to interconnect thecircuits in the component layers has been heretofore known to the art(lamination pressing method). The lamination pressing method, however,necessitates use of production facilities which are very voluminous andhighly expensive and incurs difficulty in forming fine patterns becausethe copper used in plating through-holes enters the outer layers andadds to the thickness of copper deposit.

5. In recent years, strenuous efforts are being continued for thedevelopment of a multilayer printed circuit board having conductorlayers and organic insulating films alternately built up or superposedon a substrate (build-up method) for the purpose of overcoming suchproblems as are attendant on the conventional method mentioned above. Inthis build-up method, such techniques as vacuum deposition andsputtering are generally adopted for the formation of conductor layerson insulating layers. The method, however, is at a disadvantage inbetraying inferior productivity and entailing high cost.

6. In another method of producing a printed circuit board, a “fullyadditive” technique is employed. As one of the “fully additive” method,Japanese Patent Publication No. 4-6116 discloses a method which forms aconductor layer by electroless copper plating through the medium of anadhesive layer formed on an insulating substrate. In this method, anadhesive agent composed mainly of rubber, a filler, and a thermosettingresin is used for improving the adhesive force to be exhibited by aplating film to the adhesive layer. This adhesive agent is applied tothe insulating substrate and allowed to harden thereon to produce theadhesive layer. Then, the adhesive layer is treated as with a mixture ofchromate and sulfuric acid and cleansed with hot water to give rise to asurface construction in which the depressions formed in consequence ofthe removal of filler particles and the protrusions of rubber particlesexposed in consequence of the removal of thermosetting resin areuniformly distributed. This method, however, is at a disadvantage insuffering the rubber component to persist within the adhesive layer andcause degradation of such properties as resistance to heat andelectrical insulating properties.

SUMMARY OF THE INVENTION

7. An object of the present invention, therefore, is to provide acurable resin composition which, in the production of a multilayerprinted circuit board having conductor layers severally bearing aprescribed circuit pattern and resinous insulating layers alternatelybuilt up or superposed on a substrate, accomplishes highly satisfactorybonding between the component layers by virtue of an epoxy resinexcelling in resistance to heat and electrical insulating propertieswithout requiring the resinous insulating layers to use a rubbercomponent as an essential component and enables the conductor layers toexhibit necessary adhesiveness to the insulating layer.

8. Another object of the present invention is to provide a multilayerprinted circuit board having resinous insulating layers and conductorlayers alternately built up on a substrate and joined each other withhighly satisfactory adhesive strength and enabling the resinousinsulating layers to excel in various properties such as resistance toheat and electrical insulation which are expected from a printed circuitboard and a method capable of producing the multilayer printed circuitboard inexpensively with high productivity.

9. To accomplish the objects mentioned above, in accordance with oneaspect of the present invention, there is provided a curable resincomposition which comprises (A) epoxy resins and (B) an epoxy resincuring agent as essential components thereof, the epoxy resins (A)comprising (A-1) a bisphenol A type epoxy resin having an epoxyequivalent of not less than 400 and (A-2) an epoxy resin having an epoxyequivalent of less than 400 and containing at least two epoxy groups inthe molecular unit thereof in the ratio of the epoxy resins (A-1:A-2) inthe range of from 30:70 to 90:10 by weight so that a cured coating filmformed of this resin composition can be partially decomposed ordissolved with a coarsening agent and consequently endowed with anundulating coarsened surface, and the epoxy resin curing agent (B)containing at least two active hydrogen atoms in the molecular unitthereof.

10. In a preferred embodiment of the present invention, the curableresin composition is allowed to contain, in addition to the essentialcomponents mentioned above, a rubber component in a proportion of notmore than 40 parts by weight, based on 100 parts by weight of the epoxyresins (A) and optionally further contain a filler capable of beingdecomposed or dissolved by a coarsening agent in a proportion of lessthan 70 parts by weight, desirably not more than 50 parts by weight,based on 100 parts by weight of the epoxy resins (A) mentioned above.

11. In accordance with another aspect of the present invention, there isfurther provided a method for the production of a multilayer printedcircuit board having resinous insulating layers and conductor layersseverally bearing a prescribed circuit pattern sequentially superposedon a conductor layer of a circuit board having a prescribed circuitpattern formed in advance thereon, wherein the formation of resinousinsulating layers and conductor layers comprises the steps of (a)forming the resinous insulating layer by coating a circuit board withthe curable resin composition according to the present invention andsubjecting the resultant applied layer to a thermal curing treatment,(b) treating the surface of the resultant resinous insulating layer witha coarsening agent thereby imparting an undulating coarsened surface tothe resinous insulating layer, and (c) forming a conductor layer on thecoarsened surface of the resinous insulating layer. This method for theproduction of the multilayer printed circuit board, in one concreteembodiment, further comprises the steps of boring prescribedthrough-holes in the circuit board after the formation of the outermostresinous insulating layer, treating the surfaces of the resinousinsulating layer mentioned above and the through-holes with a coarseningagent thereby imparting coarsened surfaces thereto, and thereafterforming the outermost conductor layer on the coarsened surface of theoutermost resinous insulating layer.

12. Preferably, the coating of the resinous insulating layer with theconductor layer is effected by electroless plating and/or electrolyticplating and at least one member selected from the group consisting ofoxidizing agent, alkali solution, and organic solvent is used as thecoarsening agent.

13. When the screen printing technique, for example, is used in coatingthe conductor layer of the circuit board with the resinous insulatinglayer mentioned above, it will be difficult to obtain an applied film ofdesired thickness by a single coating work. In the case of this sort,the curable resin composition of the present invention may be appliedrepeatedly or the resinous insulating layer may be formed preparatorilywith an ordinary solder resist possessing adhesiveness to the conductorlayer, desirably a curable resin composition composed mainly of an epoxyresin and an epoxy resin curing agent, and then coated with the curableresin composition according to the present invention. Thus, the appliedcoating consequently formed functions as an adhesive layer for theconductor layer to be formed afterward.

14. By the aforementioned method, there can be manufactured with highproductivity a multilayer printed circuit board which has a resinousinsulating layer and a conductor layer bearing a prescribed circuitpattern sequentially superposed on a conductor layer of a circuit boardhaving a prescribed circuit pattern formed in advance thereon, in whichthe resinous insulating layer is formed of a cured coating film of theresin composition according to the present invention, the surface ofthis resinous insulating layer which defines the interface with theconductor layer to be applied thereon is formed in an undulating roughsurface by a coarsening treatment, and the conductor layer is joined tothe resinous insulating layer through the medium of the rough surfacethereof.

BRIEF DESCRIPTION OF THE DRAWING

15. The single FIGURE is an enlarged, fragmentary cross-sectional viewschematically illustrating one example of the construction of amultilayer printed circuit board obtained by applying the method of thepresent invention to a laminated circuit board.

DETAILED DESCRIPTION OF THE INVENTION

16. The present invention is characterized prominently by the fact that,in the alternate build-up of resinous insulating layer(s) and conductorlayer(s) bearing a prescribed circuit pattern on a conductor layer of acircuit board having a prescribed circuit pattern formed in advancethereon in the manufacture of a multilayer printed circuit board, acurable resin composition which comprises (A) epoxy resins and (B) anepoxy resin curing agent or hardener as essential components thereof,the epoxy resins (A) comprising (A-1) a bisphenol A type epoxy resinhaving an epoxy equivalent of not less than 400 and (A-2) an epoxy resinhaving an epoxy equivalent of less than 400 and containing at least twoepoxy groups in the molecular unit thereof in the ratios (A-1:A-2)varying from 30:70 to 90:10 by weight so that a cured coating filmformed of this resin composition can be partially decomposed ordissolved with a coarsening agent or roughening agent and consequentlyendowed with an undulating coarsened surface, and the epoxy resin curingagent (B) containing at least two active hydrogen atoms in the molecularunit thereof, is used for the resinous insulating layer.

17. Specifically, the component (A-1) of the epoxy resins (A) mentionedabove has an epoxy equivalent of not less than 400 and contains at leastone hydroxyl group capable of being easily decomposed or dissolved by acoarsening agent. Owing to the large epoxy equivalent, it acquires arelatively low crosslink density when it is cured with an epoxy resincuring agent containing at least two active hydrogen atoms in themolecular unit thereof. Thus, it can be easily coarsened with acoarsening agent. In contrast thereto, the component (A-2) is an epoxyresin having an epoxy equivalent of less than 400 and containing atleast two epoxy groups in the molecular unit thereof and possesseseither less hydroxyl groups than the component (A-1) or no hydroxylgroup. Further, when it is cured with the epoxy resin curing agent, itacquires a high crosslink density because of its small epoxy equivalent.Thus, it is sparingly decomposed or dissolved with a coarsening agent.

18. Since the curable resin composition to be used for the formation ofthe resinous insulating layer contains as part of the epoxy resins to becontained therein 30 to 90 parts by weight, based on 100 parts by weightof the epoxy resin components, of a bisphenol A type epoxy resinexhibiting susceptibility to decomposition or dissolution by acoarsening agent and having an epoxy equivalent of not less than 400 asdescribed above, the cured coating film consequently produced reveals acontrast between the part decomposed or dissolved by the coarseningagent and the part not decomposed or dissolved, namely a contrast interms of the degree with which the coarsening agent inducesdecomposition or dissolution. By subjecting the resinous insulatinglayer which has undergone the curing treatment to a surface coarseningtreatment with a coarsening agent, therefore, an undulating surfaceconstruction can be easily formed on the surface of the cured resinousinsulating layer. The undulating coarsened surface of the resinousinsulating layer functions as an anchor for the conductor layer to beformed on the resinous insulating layer. When the conductor layer isformed thereon as by electroless plating or electrolytic plating,therefore, the adhesive strength between the resinous insulating layerand the conductor layer is improved and the multilayer printed circuitboard free from such problems as broken pattern and separation of partsand excellent in interlaminar strength can be produced. Further, sincethe resinous insulating layer is formed of a cured coating film of epoxyresin, the multilayer printed circuit board to be produced excels inresistance to heat, electrical insulating properties, and the like.

19. By using a laminated circuit board which is obtained according tothe build-up method of the present invention, the multilayer printedcircuit board to be produced preeminently excels in peel strength andresistance to the heat of soldering. The multilayer printed circuitboard to be obtained with the curable resin composition of the presentinvention is thought to be highly suitable for the production of such acircuit pattern of fine lines as has been produced by the conventionalmultilayer printed circuit board only with difficulty.

20. Examples of bisphenol A type epoxy resins (A-1) mentioned aboveinclude, but are not limited to: the products of Yuka-Shell Epoxy K.K.marketed under trademark designation of “EPIKOTE” 1001 and 1004, theproducts of Dai-Nippon Ink & Chemicals, Inc. marketed under trademarkdesignation of “EPICLON” 900 and 1050, the product of Tohto Kasei K.K.marketed under trademark designation of “Epo Tohto” YD-011, the productof The Dow Chemical Co. marketed under trademark designation of “D.E.R.”661, the products of Ciba-Geigy Ltd. marketed under trademarkdesignation of “ARALDITE” 6071 and 7072, the products of Asahi ChemicalIndustry Co., Ltd. marketed under trademark designation of “AER”-661 and-664, and the products of Sumitomo Chemical Industries Co., Ltd.marketed under trademark designation of “SUMI-Epoxy” ESA-011, ESA-012,and ESA-014. Besides, brominated bisphenol A type epoxy resins having anepoxy equivalent of not less than 400 and obtained by brominating thebisphenol A type epoxy resins may also be used.

21. It is necessary that the bisphenol A type epoxy resin (A-1) havingan epoxy equivalent of not less than 400 be contained in a proportion of30 to 90 parts by weight, preferably 50 to 80 parts by weight, in 100parts by weight of the epoxy resin components to be used. If thebisphenol A type epoxy resin (A-1) mentioned above is contained in aproportion of less than 30 parts by weight in 100 parts by weight of theepoxy resin components to be used, the curable resin composition will beat a disadvantage in that even when the cured resinous insulating layeris subjected to a surface treatment with a coarsening agent, theresinous insulating layer does not easily acquire an undulating surfacestructure capable of amply functioning as an anchor for the conductorlayer to be subsequently formed thereon. Conversely, if the proportionexceeds 90 parts by weight, the produced resinous insulating layer willentail a problem of deficiency in resistance to soldering heat.

22. As the epoxy resin for the component (A-2) mentioned above, any ofthe known epoxy compounds (including epoxy oligomers) which have anepoxy equivalent of less than 400 and contain at least two epoxy groupsin the molecular unit thereof can be used. As concrete examples of theepoxy resin usable herein, bisphenol A type epoxy resins such as theproduct of Yuka-Shell Epoxy K.K. marketed under trademark designation of“EPIKOTE” 828, the product of Dai-Nippon Ink & Chemicals, Inc. marketedunder trademark designation of “EPICLON” 840, and the product of TohtoKasei K.K. marketed under trademark designation of “Epo Tohto” YD-128,and bisphenol F type epoxy resins, bisphenol S type epoxy resins, phenolnovolak type epoxy resins, cresol novolak type epoxy resins, alicyclicepoxy resins, triazine nucleus-containing epoxy resins, biphenyl typeepoxy resins, brominated epoxy resins, dimer acid-modified epoxy resins,trihydroxyphenyl methane type epoxy resins, hydrogenated bisphenol Atype epoxy resins, glycidyl amine type epoxy resins, tetraphenylolethane type epoxy resins, and heterocyclic epoxy resins may be cited.

23. As concrete examples of the epoxy resin curing agent (B) whichcontains at least two active hydrogen atoms in the molecular unitthereof and is used essentially in conjunction with the epoxy resinsmentioned above in the curable resin composition of the presentinvention, amines, aminopolyamide resins, and dicyandiamide may becited. The epoxy resin curing agent which contains at least two activehydrogen atoms in the molecular unit thereof is used for the purpose ofobtaining a three-dimensionally cross-linked and cured film composed ofthe epoxy resins (A-1) and (A-2) mentioned above and consequentlyendowed with a contrast in terms of the degree with which the film isdecomposed or dissolved with the coarsening agent.

24. Examples of the epoxy resin curing agents mentioned above include,but are not limited to: amines such as diethylene triamine, triethylenetetramine, isophorone diamine, metaxylylene diamine, m-phenylenediamine, p-phenylene diamine, 4,4′-diaminodiphenyl methane,4,4′-diaminodiphenyl sulfone, 4,4′-diaminodiphenyl ether, andaniline-formalin resin; aminopolyamide resins which are condensates ofdimer acids with diethylene triamine, triethylene tetramine, or thelike; and latent curing agents such as dicyandiamide, dihydrazideadipate, and hydrazide sebacate.

25. The amount of the epoxy resin curing agent to be used in the curableresin composition of the present invention is such that the activehydrogen content in the curing agent is in the range of from 0.5 to 1.5equivalent weights, preferably from 0.8 to 1.2 equivalent weights perthe equivalent weight of the epoxy group content in the epoxy resincomponents when the curing agent is an amino group-containing compoundsuch as an amine and a polyamide resin or in the range of from 0.3 to0.7 equivalent weight when the curing agent is dicyandiamide.

26. The curable resin composition of the present invention, whennecessary, may incorporate therein any of well-known curing acceleratorsor promoters for the purpose of promoting the curing reaction of thecomposition. Examples of the curing promoters include, but are notlimited to: tertiary amines such as triethyl amine, tributyl amine,dimethylbenzyl amine, diethylbenzyl amine,4-(dimethyl-amino)-N,N-dimethylbenzyl amine,4-methoxy-N,N-dimethylbenzyl amine, and 4-methyl-N,N-dimethylbenzylamine; quaternary ammonium salts such as benzyltrimethylammoniumchloride and benzyltriethylammonium chloride; phosphines such astriethylphosphine and triphenylphosphine; phosphonium salts such asn-butyltriphenylphosphonium bromide; imidazoles such as imidazole,2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole,2-phenylimidazole, 1-(2-cyanoethyl)-2-ethyl-4-methylimidazole, ororganic acid salts thereof; guanamines such as acetoguanamine andbenzoguanamine. The preferred curing promoters are imidazoles andphosphines.

27. The curable resin composition of the present invention, whennecessary, may contain a rubber component. In the film obtained afterthe surface coarsening treatment, this rubber component functions as astress alleviating agent and improves the adhesive strength of the film.As concrete examples of the rubber component, polybutadiene rubbers suchas, for example, the product of Idemitsu Kosan Co., Ltd. marketed underproduct code of R-45HT; urethane-modified, maleated, epoxy-modified, or(meth)acryloyl-modified various polybutadiene rubber derivatives suchas, for example, the epoxy-modified product of Idemitsu Kosan Co., Ltd.marketed under product code of R-45EPI; nitrile rubbers such as, forexample, the products of JSR K.K. marketed under product codes of N280and N230S; CTBN (carboxy terminated acrylonitrile rubber) such as, forexample, the product of Ube Industries, Ltd. marketed under product codeof 1300-31; and CTBN-modified epoxy resins such as, for example, theproducts of Tohto Kasai K.K. marketed under product codes of YR-102 andYR-450 may be cited.

28. Further in the curable resin composition of the present invention,the depth of the undulation of the surface of the resinous insulatinglayer can be increased and the adhesive strength of this layer with theconductor layer can be further enhanced by using a filler capable ofbeing decomposed or dissolved by the coarsening agent in combinationwith the components of the composition mentioned above. If the filler isincorporated in the composition in a proportion of 70 parts by weight ormore, based on 100 parts by weight of the epoxy resins, it will giverise to voids in the cured film or degrade the electrical insulatingproperties of the film. It is necessary that the content of the fillerbe less than 70 parts by weight, preferably not more than 50 parts byweight.

29. The filler mentioned above is known in two kinds; organic filler andinorganic filler. As concrete examples of the organic filler, powderedepoxy resins such as, for example, the product of Nissan ChemicalsIndustries, Ltd. marketed under trademark designation of “TEPIC”;melamine resins; benzoguanamine resins such as, for example, theproducts of Nippon Shokubai K.K. marketed under product codes of M-30,S, and MS; urea resins; and cross-linked acryl polymers such as, forexample, the products of Soken Kagaku K.K. marketed under product codesof MR-2G and MR-7G and the product of Sekisui Plastics Co., Ltd.marketed under trademark designation of “Techpolymer” may be cited. Asconcrete examples of the inorganic filler, magnesium oxide, calciumcarbonate, zirconium silicate, zirconium oxide, calcium silicate, andcalcium hydroxide may be cited.

30. The curable resin composition of the present invention, whennecessary, may incorporate therein any of organic solvents. Examples ofthe organic solvents include, but are not limited to: ketones such asmethylethyl ketone and cyclohexanone; aromatic hydrocarbons such astoluene and xylene; cellosolves such as cellosolve and butyl cellosolve;carbitols such as carbitol and butyl carbitol; and acetates such asethyl acetate, butyl acetate, cellosolve acetate, butyl cellosolveacetate, carbitol acetate, and butyl carbitol acetate. These organicsolvents may be used either singly or in the form of a combination oftwo or more members.

31. Further, the curable resin composition of the present invention mayincorporate therein, depending on the desired properties thereof, a wellknown and widely used filler such as barium sulfate, silicon sulfide,talc, clay, silica, bentonite, kaolin, glass fiber, carbon fiber, mica,asbestos, and metal powder; a color pigment such as phthalocyanine blue,phthalocyanine green, titanium oxide, and carbon black; and othervarious additives such as an anti-foaming agent, anadhesiveness-imparting agent, and a leveling agent.

32. The manufacture of the multilayer printed circuit board of thepresent invention begins with the application of the curable resincomposition comprising the components mentioned above to the uppersurface of a conductor layer of a circuit board bearing a prescribedcircuit pattern by any of the known coating methods such as, forexample, the screen printing method, spray coating method, and curtaincoating method. Some of these coating methods may fail to deposit thecomposition in a desired thickness in one coating operation. In thiscase, the application of the composition may be repeated until thedesired thickness is obtained. When the coating is repeated, the curableresin composition of the present invention may be used all by itself.Otherwise, some other curable resin composition possessing highlydesirable adhesiveness to copper may be applied as an undercoat and,thereafter, the curable resin composition of the present invention maybe used as the uppermost layer of coating.

33. After the resinous insulating layer is produced in a desiredthickness by the coating as described above, it is cured by a heattreatment. It is then perforated, when necessary, to form through-holesand other holes therein and, thereafter, subjected to a surfacecoarsening treatment with a coarsening agent selected from amongoxidizing agents, aqueous alkali solutions, and organic solvents toprovide the resinous insulating layer and the through-hole parts withhighly satisfactory undulating coarsened surfaces. Subsequently, thecoarsened surfaces of the resinous insulating layer and through-holeparts thus obtained are coated with a conductor layer by means ofelectroless plating or electrolytic plating. The resinous insulatinglayer, when necessary, is subjected again to a heat treatment (annealingof plate layer) to enhance the crosslink density of the resinousinsulating layer and alleviate the stress therein. Thereafter, aspopularly practiced, the conductor layer on the surface of the resinousinsulating layer is patterned to form a desired circuit. The proceduredescribed above may be repeated, when desired, to implement alternatebuild-up of resinous insulating layers and conductor layers severallybearing a prescribed circuit pattern. It is provided, however, that theformation of the through-holes be carried out after the formation of theoutermost resinous insulating layer. Optionally, the formation of theconductor circuit may be effected by the “fully additive” techniqueusing permanent plating resist.

34. In the method for the production of the multilayer printed circuitboard described above, the temperatures of the heat treatments which areperformed between the formation of the resinous insulating layer and thecoarsening of surface are desired to be in the range of from 130 to 170°C. Properly, the duration of the heat treatment is approximately 60 to120 minutes when the temperature is 130° C. or 5 to 20 minutes when thetemperature is 170° C., preferably 15 to 60 minutes when the temperatureis in the range of from 140° C. to 160° C. If the temperature of heattreatment is less than 130° C., the resinous insulating layer will notbe cured enough to withstand the drilling work performed for theformation of through-holes or the punching work otherwise performed bythe use of a metal die, which in turn results in occurrence of smear orflaw. In consideration of the possibility of the substrate beingshrunken or warped by heat, the upper limit of the temperature of heattreatment should be 170° C., though variable with the kind of thematerial to be used for the substrate.

35. Now, embodiments of the present invention will be described indetail below with reference to the accompanying drawing.

36. First, one example of the construction of a multilayer printedcircuit board to be manufactured by the method of the present inventionwill be described. The multilayer printed circuit board is obtained byforming a first conductor layer 1 bearing a prescribed circuit patternthrough the medium of a first resinous insulating layer 2 on the uppersurface of a laminated circuit board A and forming a sixth conductorlayer 11 bearing a prescribed circuit pattern through the medium of asecond resinous insulating layer 10 on the undersurface of the laminatedcircuit board A as shown in FIGURE. The laminated circuit board Amentioned above results from sequential superposition and union of threesubstrates 4, 6, and 8 made of glass-fiber-reinforced epoxy resin. Asecond conductor layer 3 bearing a prescribed circuit pattern islaminated on the surface of the substrate 4 opposite the junctionboundary thereof with the substrate 6, a third conductor layer 5 bearinga prescribed circuit pattern is interposed between the substrate 4 andthe substrate 6, a fourth conductor layer 7 bearing a prescribed circuitpattern is interposed between the substrate 6 and the substrate 8, and afifth conductor layer 9 bearing a prescribed circuit pattern is formedon the surface of the substrate 8 opposite the junction boundary thereofwith the substrate 6. Thus, the laminated circuit board A mentionedabove is possessed of a total of four conductor layers.

37. The multilayer printed circuit board described above is furtherprovided with a plated-through hole 20 which electrically connects aconnection part 1 a of the first conductor layer 1 to a connection part7 a of the fourth conductor layer 7 and a connection part 11 a of thesixth conductor layer 11. The plated-through hole 20 is so formed as topierce the first resinous insulating layer 2, the laminated circuitboard A, and the second resinous insulating layer 10 and also runsthrough the central parts of the connection parts 1 a, 7 a, and 11 afrom the connection part 1 a of the first conductor layer 1 through theconnection part 11 a of the sixth conductor layer 11. Further, theplated-through hole 20 has a conducting material disposed on the innerwall surface of the through-hole 21 to interconnect the connection parts1 a, 7 a, and 11 a electrically.

38. Incidentally, the multilayer printed circuit board is provided withblind via holes which electrically interconnect between the connectionparts of the first conductor layer 1 and the second conductor layer 3and between the connection parts of the sixth conductor layer 11 and thefifth conductor layer 9. These blind via holes roughly resemble theplated-through hole 20 in construction and function to interconnect theconnection parts electrically. They are omitted from illustrationherein. These blind via holes can be bored by any of the conventionalmethods using a laser beam, sand blast, or the like. The presentinvention does not discriminate these blind via holes on account of themethod to be employed for boring them.

39. Now, the method for embodying the present invention in the laminatedcircuit board A mentioned above will be described. The resinousinsulating layers 2 and 10 are formed by applying the curable resincomposition of the present invention by a suitable method such as, forexample, the screen printing method, spray coating method, or curtaincoating method to a second conductor layer 3 and a fifth conductor layer9 bearing prescribed circuit patterns and formed with copper foil on theopposite surfaces of the laminated circuit board A and then thermallycuring the applied layers of the composition.

40. Then, a through-hole 21 is formed in such a manner as to pierce theresinous insulating layers 2 and 10 and the laminated circuit board A.This through-hole 21 can be formed by a suitable means such as a drill,a metal punch, or a laser beam.

41. Thereafter, the resinous insulating layers 2 and 10 are subjected toa surface coarsening treatment using a coarsening agent.

42. The coarsening agents which can be used for this surface coarseningtreatment include oxidizing agents such as potassium permanganate,potassium bichromate, ozone, hydrochloric acid, sulfuric acid, nitricacid, and hydrofluoric acid; organic solvents such asN-methyl-2-pyrrolidone, N,N-dimethyl formamide, dimethyl sulfoxide, andmethoxy propanol; and alkalis such as sodium hydroxide and potassiumhydroxide, for example. When an oxidizing agent, for example, is adoptedas the coarsening agent, the resinous insulating layers 2 and 10 areswelled with such an organic solvent as is mentioned above and thensubjected to the surface coarsening treatment using the oxidizing agent.By this surface coarsening treatment, an undulating surface constructioncan be easily formed on the resinous insulating layers 2 and 10 and thewall surrounding the through-hole 21.

43. Then, conductor layers are formed on the surfaces of the resinousinsulating layers 2 and 10 by electroless plating, electrolytic plating,or the combination of electroless plating and electrolytic plating. As aresult, the conductor layers are formed not only on the surfaces of theresinous insulating layers 2 and 10 but also on the entire surfaces ofthe through-hole 21 and the blind holes. Subsequently, etching resistsare printed on the surfaces of the conductor layers by the screenprinting or offset printing, the exposed portions of the conductorlayers are etched, and thereafter the etching resists are removed. Inthis manner, prescribed circuit patterns are formed on the conductorlayers overlying the surfaces of the resinous insulating layers 2 and 10to complete the first conductor layer 1 and the sixth conductor layer 11as shown in FIGURE. At this time, a plating layer is formed also on theinner surface of the through-hole 21 as mentioned above. As a result,this plating layer constitutes itself the plated-through hole 20 whichelectrically interconnect the connection part la of the first conductorlayer 1, the connection part 7 a of the fourth conductor layer 7, andthe connection part 11 a of the sixth conductor layer 11 in themultilayer printed circuit board mentioned above.

44. Though the embodiment described thus far represents a case offorming resinous insulating layers and conductor layers on a laminatedcircuit board, the present invention can be likewise embodied by using aone-sided circuit board or a double-sided circuit board in the place ofthe laminated circuit board.

45. Now, working examples and comparative examples which havespecifically demonstrated the effect of the present invention will bedescribed below.

EXAMPLES 1 THROUGH 6 AND COMPARATIVE EXAMPLES 1 THROUGH 4

46. Curable resin compositions were prepared in the formulas (in partsby weight) shown in Tables 1 and 2. In the preparation of the curableresin compositions of these working examples and comparative examples,EPIKOTE 1001 and YDCN-704 were used in the form of resinous solutionsprepared in advance by dissolving the respective epoxy resins incarbitol acetate at normal room temperature. Each resinous solution had0.5 part by weight of KS-66 as an anti-foaming agent and 3 parts byweight of AEROSIL #200 (trademark of Nippon Aerosil Co., Ltd. for finelypowdered silicon oxide) as a printability enhancer added thereto anddispersed therein and was kneaded with a three-roll kneader. It wasfurther diluted with carbitol acetate to a point where the dilutedresinous solution could be applied to a surface of a circuit board byscreen printing.

47. Examples 1 through 4 used epoxy resins in varied proportions andExamples 5 and 6 used a filler and a rubber component additionally. Asrespect the comparative examples, Comparative Example 1 used anotherepoxy resin instead of a bisphenol A type epoxy resin having an epoxyequivalent of not less than 400, Comparative Examples 2 and 4 containeda bisphenol A type epoxy resin having an epoxy equivalent of not lessthan 400 in proportions deviating from the range defined by the presentinvention, and Comparative Example 3 used a filler and a rubbercomponent in addition to an epoxy resin other than a bisphenol A typeepoxy resin having an epoxy equivalent of not less than 400. Thesecurable resin compositions were each applied by screen printing to aboard having a circuit formed in advance thereon, thermally cured at150° C. for 30 minutes, and then subjected to a surface coarseningtreatment using a coarsening agent (oxidizing agent, solvent, or alkalisolution). The surface coarsening treatment was carried out by firstswelling a sample with a mixture of 20% by weight of1-methoxy-2-propanol, 5% by weight of sodium hydroxide, and 75% byweight of water, coarsening the swelled sample with an oxidizing agent(aqueous 6-7% KMnO₄ solution), and rinsing the sample in an aqueous0.1-0.2% sulfuric acid solution, and the roughness of the coarsenedsurface was determined. The coarsened surface of the sample wassubjected to electroless copper plating and electrolytic copper platingand thereafter to annealing. The resultant sample was tested for coatingproperties to evaluate it as a copper-plated circuit board. The resultsare shown in Tables 1 and 2. TABLE 1 Examples Composition 1 2 3 4 5 6EPIKOTE^(a)) 1001 90 70 50 30 50 50 EPIKOTE^(b)) 828 10 — 20 20 20 20DEN^(c)) −431 — 30 30 30 30 30 YDCN^(d)) −704 — — — 20 — — Dicyandiamide 5  5  5  5  5  5 2E4MZ^(e))  5  5  5  5  5  5 CTBN 1300-31^(f)) — — — —— 20 Calcium carbonate — — — — 50 50 Roughness after   3.2   4.0   3.8  3.1   5.2   4.9 coarsening treatment^(g)) (μm) Blister after annealingAbsence Absence Absence Absence Absence Absence of plate layer Peelstrength   0.8   1.2   1.0   0.7   1.4   1.5 (kg/cm) Soldering heatresistance ◯ ◯ ◯ ◯ ◯ ◯ (260° C. × 30″)

48. TABLE 2 Comparative Examples Composition 1 2 3 4 EPIKOTE^(a)) 1001 —20 — 100    EPIKOTE^(b)) 828 30 30 30 — DEN^(c)) −431 50 50 50 —YDCN^(d)) −704 20 — 20 — Dicyandiamide  5  5  5 5   2E4MZ^(e))  5  5  55   CTBN 1300-31^(f)) — — 20 — Calcium carbonate — — 50 — Roughnessafter 1.0   1.5   1.8   2.3 coarsening treatment^(g)) (μm) Blister afterPresence Presence Absence Absence annealing of plate layer Peel strength— — 0.2   0.6 (kg/cm) (impossible (impossible to measure) to measure)Soldering heat × × × × resistance (260° C. × 30″)

49. The marks indicated in Tables 1 and 2 as results of rating ofproperties are based on the following scales.

50. Resistance to heat of soldering:

51. A given sample was left floating on a solder bath kept at 260° C.for 3 cycles each of 10 seconds (total 30 seconds). After the floating,the sample was examined with respect to “blister” of the copper platelayer.

52. ◯: Absence of discernible blister

53. ×: Occurence of blister

54. The peel strength of the copper plate layer was determined inaccordance with the method specified in JIS (Japanese IndustrialStandards) C-6481 except that the peeling velocity of 100 mm/min. wasemployed.

55. It is clearly noted from Tables 1 and 2 that the surface coarseningtreatment produced no sufficient roughness in Comparative Examples 1 and3 using curable resin compositions which did not contain a bisphenol Atype epoxy resin having an epoxy equivalent of not less than 400, thatthe sample of Comparative Example 1 was blistered and the sample ofComparative Example 3 was not blistered after annealing of the platelayer, that the samples of Comparative Examples 1 and 3 were bothblistered in a test for resistance to the heat of soldering (260° C.×30seconds), and that the sample of Comparative Example 1 did not toleratea test for peel strength. With respect to Comparative Examples 2 and 4using curable resin compositions which contained a bisphenol A typeepoxy resin having an epoxy equivalent of not less than 400 inproportions deviating from the range defined by the present invention,it is noted that the sample of Comparative Example 2 containingbisphenol A type epoxy resin having an epoxy equivalent of not less than400 in a proportion deviating from the lower limit of the rangespecified by the present invention was blistered and the sample ofComparative Example 4 containing the bisphenol A type epoxy resin in aproportion deviating from the upper limit of the range was not blisteredafter the annealing of the plate layer, that both the samples wereblistered in a test for resistance to the heat of soldering and wereheavily deficient in peel strength, and that the sample of ComparativeExample 2 did not tolerate the test for peel strength. In contrast tothese comparative examples, the samples of Examples 1 through 6 usingcurable resin compositions in accordance with the present invention werefound to possess outstanding peel strength and surface roughness. Theseexcellent results may be logically explained by a supposition that theircoarsened surfaces had an unusually fine undulating profile andconsequently enjoyed improved peel strength. Further, the samples of allthe working examples were stable to resist the heat of soldering andcould be used satisfactorily as copper-clad laminates. They demonstratethat the incorporation of a bisphenol A type epoxy resin having an epoxyequivalent of not less than 400 in proportions in the range of from 30to 90 parts by weight per 100 parts by weight of the epoxy resincomponents in a curable resin composition contributed to improve peelstrength and resistance to the heat of soldering. It is found bycomparing Example 3 with Examples 5 and 6 that the curable resincomposition additionally incorporating a filler capable of beingdecomposed or dissolved by a coarsening agent enjoyed greater roughnessof the surface of the resinous insulating layer after the surfacecoarsening treatment and acquired still greater peel strength.

56. While certain specific embodiments and working examples have beendisclosed herein, the invention may be embodied in other specific formswithout departing from the spirit or essential characteristics thereof.The described embodiments and examples are therefore to be considered inall respects as illustrative and not restrictive, the scope of theinvention being indicated by the appended claims rather than byforegoing description and all changes which come within the meaning andrange of equivalency of the claims are, therefore, intended to beembraced therein.

What is claimed is:
 1. A curable resin composition, which comprises: (A)epoxy resins and (B) an epoxy resin curing agent, said epoxy resins (A)comprising (A-1) a bisphenol A type epoxy resin having an epoxyequivalent of not less than 400 and (A-2) an epoxy resin having an epoxyequivalent of less than 400 and containing at least two epoxy groups inthe molecular unit thereof in the ratio of said epoxy resins (A-1:A-2)in the range of from 30:70 to 90:10 by weight so that a cured coatingfilm formed of the resin composition can be partially decomposed ordissolved with a coarsening agent and consequently endowed with anundulating coarsened surface, and said epoxy resin curing agent (B)containing at least two active hydrogen atoms in the molecular unitthereof.
 2. The composition according to claim 1 , which furthercomprises a rubber component in a proportion of not more than 40 partsby weight, based on 100 parts by weight of said epoxy resins (A).
 3. Thecomposition according to claim 1 , which further comprises a fillercapable of being decomposed or dissolved by said coarsening agent in aproportion of less than 70 parts by weight, based on 100 parts by weightof said epoxy resins (A).
 4. The composition according to claim 1 ,wherein said epoxy resin (A-2) has no hydroxyl group in the molecularunit thereof.
 5. The composition according to claim 1 , wherein saidepoxy resin (A-2) has less hydroxyl groups than said bisphenol A typeepoxy resin (A-1).
 6. The composition according to claim 1 , whereinsaid epoxy resin (A-2) is selected from the group of bisphenol A typeepoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxyresins, phenol novolak type epoxy resins, cresol novolak type epoxyresins, alicyclic type epoxy resins, triazine nucleus-containing epoxyresins, biphenyl type epoxy resins, brominated epoxy resins, dimeracid-modified epoxy resins, trihydroxyphenyl methane type epoxy resins,hydrogenated bisphenol A type epoxy resins, glycidyl amine type epoxyresins, tetraphenylol ethane type epoxy resins, and heterocyclic typeepoxy resins.
 7. The composition according to claim 1 , wherein saidepoxy resin curing agent (B) is selected from the group of amines,amino-polyamide resins, dicyan diamide, dihydrazide adipate, andhydrazide sebacate.
 8. The composition according to claim 1 , whereinsaid epoxy resin curing agent (B) is amino groups-containing compoundand is incorporated in the composition in a proportion such that theactive hydrogen content in said compound is in the range of from 0.5 to1.5 equivalent weights per one equivalent weight of epoxy group contentin said epoxy resins (A).
 9. The composition according to claim 1 ,wherein said epoxy resin curing agent (B) is dicyandiamide and isincorporated in the composition in a proportion such that the activehydrogen content in dicyandiamide is in the range of from 0.3 to 0.7equivalent weight per one equivalent weight of epoxy group content insaid epoxy resins (A).
 10. The composition according to claim 2 ,wherein said rubber component is selected from the group ofpolybutadiene rubber, urethane-modified polybutadiene rubber, maleatedpolybutadiene rubber, epoxy-modified polybutadiene rubber,(meth)acryloyl-modified polybutadiene rubber, nitrile rubber, carboxyterminated acrylonitrile rubber (CTBN), and CTBN-modified epoxy resin.11. The composition according to claim 3 , wherein said filler is anorganic filler selected from the group of powdered epoxy resins,melamine resins, benzoguanamine resins, urea resins, and cross-linkedacryl polymers.
 12. The composition according to claim 3 , wherein saidfiller is an inorganic filler selected from the group of magnesiumoxide, calcium carbonate, zirconium silicate, zirconium oxide, calciumsilicate, and calcium hydroxide.
 13. The composition according to claim1 , which further comprises an organic solvent.
 14. The compositionaccording to claim 1 , which further comprises an epoxy resin curingpromotor.
 15. The composition according to claim 1 , which furthercomprises at least one additive selected from the group of colorpigments, anti-foaming agents, and leveling agents.
 16. A method for theproduction of a multilayer printed circuit board having resinousinsulating layers and conductor layers severally bearing a prescribedcircuit pattern sequentially superposed on a conductor layer of acircuit board having a prescribed circuit pattern formed in advancethereon, wherein the formation of said resinous insulating layers andconductor layers comprising the steps of: (a) forming a resinousinsulating layer on said circuit board by coating the circuit board witha curable resin composition and subjecting the resultant coating to athermal curing treatment, said curable resin composition being acomposition which comprises (A) epoxy resins and (B) an epoxy resincuring agent, said epoxy resins (A) comprising (A-1) a bisphenol A typeepoxy resin having an epoxy equivalent of not less than 400 and (A-2) anepoxy resin having an epoxy equivalent of less than 400 and containingat least two epoxy groups in the molecular unit thereof in the ratio ofsaid epoxy resins (A-1:A-2) in the range of from 30:70 to 90:10 byweight so that a cured coating film formed of the resin composition canbe partially decomposed or dissolved with a coarsening agent andconsequently endowed with an undulating coarsened surface, and saidepoxy resin curing agent (B) containing at least two active hydrogenatoms in the molecular unit thereof; (b) treating a surface of theresultant resinous insulating layer with a coarsening agent therebyimparting an undulating coarsened surface to the resinous insulatinglayer; and (c) forming a conductor layer on the coarsened surface ofsaid resinous insulating layer.
 17. The method according to claim 16 ,which further comprises steps of boring a prescribed through-hole in thecircuit board after the formation of an outermost resinous insulatinglayer, treating surfaces of said resinous insulating layer and saidthrough-hole with a coarsening agent thereby imparting coarsenedsurfaces thereto, and thereafter forming an outermost conductor layer onthe coarsened surface of said outermost resinous insulating layer. 18.The method according to claim 16 , wherein said step (a) comprisespreparatorily forming a resinous insulating layer on said conductorlayer of the circuit board with a resin composition containing an epoxyresin and an epoxy resin curing agent as main components thereof andthen forming a resinous insulating layer with said curable resincomposition.
 19. The method according to claim 16 , wherein theapplication of a coating of the conductor layer to the resinousinsulating layer is carried out by any one of electroless plating,electrolytic plating and the combination thereof.
 20. The methodaccording to claim 16 , wherein said coarsening agent is at least onemember selected from the group of oxidizing agents, alkali solutions,and organic solvents.
 21. A multilayer printed circuit board having aresinous insulating layer and a conductor layer bearing a prescribedcircuit pattern sequentially superposed on a conductor layer of acircuit board having a prescribed circuit pattern formed in advancethereon, wherein said resinous insulating layer being formed of a curedcoating film of a resin composition which comprises (A) epoxy resins and(B) an epoxy resin curing agent, said epoxy resins (A) comprising (A-1)a bisphenol A type epoxy resin having an epoxy equivalent of not lessthan 400 and (A-2) an epoxy resin having an epoxy equivalent of lessthan 400 and containing at least two epoxy groups in the molecular unitthereof in the ratio of said epoxy resins (A-1:A-2) in the range of from30:70 to 90:10 by weight so that a cured coating film formed of theresin composition can be partially decomposed or dissolved with acoarsening agent and consequently endowed with an undulating coarsenedsurface, and said epoxy resin curing agent (B) containing at least twoactive hydrogen atoms in the molecular unit thereof; a surface of saidresinous insulating layer which defines an interface with the conductorlayer to be applied thereon being formed in an undulating rough surfaceby a coarsening treatment, and said conductor layer being joined to saidresinous insulating layer through the medium of said rough surfacethereof.
 22. A method of laminating a resinous insulating layer and aconductor layer on a conductor layer of a circuit board, the methodcomprising the steps of: providing a cirduit board having a prescribedcircuit pattern formed in advance thereon; providing a curable resincomposition which comprises (A) epoxy resins and (B) an epoxy resincuring agent, said epoxy resins (A) comprising (A-1) a bisphenol A typeepoxy resin having an epoxy equivalent of not less than 400 and (A-2) anepoxy resin having an epoxy equivalent of less than 400 and containingat least two epoxy groups in the molecular unit thereof in the ratio ofsaid epoxy resins (A-1:A-2) in the range of from 30:70 to 90:10 byweight so that a cured coating film formed of the resin composition canbe partially decomposed or dissolved with a coarsening agent andconsequently endowed with an undulating coarsened surface, and saidepoxy resin curing agent (B) containing at least two active hydrogenatoms in the molecular unit thereof; applying said resin composition onsaid circuit board to form a coating film thereon; thermally curing saidcoating film thereby forming a resinous insulating layer laminated onsaid circuit board; treating a surface of said resinous insulating layerwith a coarsening agent thereby imparting an undulating coarsenedsurface to the resinous insulating layer; and plating a conductor layeron said coarsened surface of the resinous insulating layer.
 23. Themethod according to claim 22 , wherein said plating of a coating of theconductor layer to the resinous insulating layer is carried out by anyone of electroless plating, electrolytic plating and the combinationthereof.
 24. The method according to claim 22 , wherein said coarseningagent is at least one member selected from the group of oxidizingagents, alkali solutions, and organic solvents.